This invention relates to a feed control device for an electronically controlled zigzag sewing machine, and more particularly to a feed control device for a feed device which is directly driven by a pulse motor.
In a conventional electronically controlled zigzag sewing machine, generally, the needle bar reciprocating mechanism and feed dog vertical motion mechanism for fabric feed were driven by the main motor, while the needle bar swinging mechanism and feed dog driving mechanism were driven by individual pulse motors.
Recently, zigzag sewing machines capable of controlling the fabric feed accurately based on the latest required feed stroke by directly driving the feed dog driving mechanism by an independent pulse motor to simplify the driving system have been proposed.
For example, the U.S. Pat. No. 4,286,532 discloses a feed device driven independently from the sewing machine upper shaft, in which the rotation phase of the sewing machine upper shaft is detected by an upper shaft phase signal generator, the rotation phase of the feed shaft of the feed dog is predetermined to correspond to the rotation phase of the upper shaft, and the rotation phase of the feed shaft is controlled to agree with the predetermined rotation phase depending on an upper shaft phase signal, so that the end timing of fabric feed is controlled constantly.
In this feed device, although the end timing of fabric feed can be adequately controlled, it requires the upper shaft phase signal generator for detecting the rotation phase of the sewing machine upper shaft and electric signal means for setting the rotation phase of the feed shaft so as to correspond with the rotation phase of the upper shaft, and the constitution of the device is complicated, and the manufacturing cost is higher.
The present applicant, therefore, previously proposed a feed device of a sewing machine (the U.S. Ser. No. 06/931 853) now U.S. Pat. No. 4,721,050 in which the start timing and end timing of fabric feed are controlled constantly regardless of the sewing machine speed, by calculating the period of the command pulses of the feed pulse motor according to the speed signal corresponding to the actual speed or set speed of the main motor and the feed stroke signal for feeding the fabric.
In the feed device of the sewing machine according to previous application, the period of the command pulses of the feed pulse motor is controlled longer when the speed of main motor is low or feed stroke is small, while the period of the command pulses is controlled shorter in case of high speed or large feed stroke, and by thus constantly controlling the end timing of fabric feed, the tightening of the needle thread by the thread take-up lever is kept constant.
However, only by controlling the end timing of fabric feed to be constant regardless of the rotating speed of the main motor, tightening of the thread in high speed operation is insufficient, and the tension of needle is unstable.
Even in the conventional sewing machine without controlling of the end timing of fabric feed constant, it is empirically known that the thread tightening is insufficient at high speed, and therefore an experienced operator adjusts to intensify the spring force of the tension disc in high speed operation.
Considering the reason of the insufficient tightening in high speed operation, the thread tightening time is extremely short in high speed operation, and the ascending speed of the thread take-up lever is extremely raised so that the thread cannot move smoothly toward the tension disc through the guide hole of the thread take-up lever when tightening the needle thread, thereby causing the needle thread to be somewhat supplied through the tension disc.
Furthermore, when the start timing of fabric feed is controlled constant regardless of the speed of the main motor or fabric feed stroke, the feed end timing is earlier when the speed is low or the feed stroke is small, which may result in disturbance of the thread tension.